This invention relates generally to signalling systems for use by the infirm, elderly, etc. to summon aid or assistance as the need arises.
Prior alerting systems, such as those employed in hospital or convalescent facilities, were of the type having connecting leads between an alarm circuit located at a remote location such as a nurse's station, etc., and an actuator button at the bedside of the patient. In such installations, the placement of these leads presented a problem, particularly if the system was installed after the construction of the building was complete. In addition, the cords associated with the actuator button were awkward in that they tended to become tangled in furniture, twisted, kinked, etc. Such systems lacked any degree of portability or flexibility, and were thus useable only by personnel confined in bed or in a single room.
Other signalling systems employing transmitters and receivers have been proposed and produced. One of the problems encountered in such devices was that stray signals (both fundamental and harmonics thereof) from police radios, aircraft, etc. were detected by the receiver, thus often triggering a false alarm. To overcome this, modulation was added to the system such that only a signal on the proper carrier frequency having the proper modulating frequency superimposed thereon would trigger the alarm. Such systems are employed in automatic garage door openers which are currently being manufactured and sold.
The construction of a receiver which responds to only a particular modulating frequency means a multiplicity of additional components as required by the selective circuitry. Typically, tuned filters are employed after the detector, such filters blocking all signals except those on the desired (audio) frequency. Unfortunately, filter components (inductors, capacitors) in the audio range tend to be rather large and bulky. Filters of the active type are tricky to adjust and maintain. As a result, prior transmitter/receiver systems have been rather expensive and complex, and in some cases the reliability has been poor.